Roman Pantheonissa naturally wonders how wonderful that temple is inside.
Another more everyday subject of wonder is how on earth the building’s concrete has lasted almost 2,000 years. The Pantheon was completed in the late 120s.
Meanwhile, millions and millions of tons of concrete have been produced worldwide. Concrete construction has penetrated everyday life in every area: houses, bridges and architecture in general.
We already generally talk about the stylistic direction of concrete, concrete brutalism after the Second World War.
However, modern concrete seems to corrode faster than the best concrete made in ancient Rome. Why?
This has now been studied with precise instruments. It seems that the concrete of Roman times knows how to fix and improve itself. If cracks appear in the concrete, water will eventually end up in them. Then, patching substances containing calcium compounds are released from the concrete.
The researchers collected ancient concrete samples from the Privernum archaeological site. In a piece of concrete two centimeters in diameter, the element calcium is marked in red, silicon in blue, and aluminum in green. Calcium from lime improves concrete if it starts to corrode. Picture: with
Pantheonissa is still the largest unsupported concrete arched roof in the world. The diameter of the roof is 43.44 meters.
Of course, the building has been and is being renovated over time, but the concrete roof has never once collapsed.
The dome is cast from concrete, which of course contains pure lime, one of the basic ingredients of concrete. In addition, pumice and volcanic ash, so-called pozzolan, have been mixed into the concrete.
There is also sinter, which is a substance precipitated from minerals dissolved in water.
Roman the strength of concrete has been known for a long time. It has usually been explained by pozzolan.
It is a fine blend of volcanic ash from the Bay of Naples area. It was commonly used in Roman-era concrete construction, says the Swedish publication Ny Teknik.
Pozzolana got its name from Italy Pozzuolin according to the city near which there is a lot of good ash. Pozzolan was shipped from there up to 2,000 kilometers away as a building material.
So the ingredients of concrete once had a different composition than in it in concrete, which now commonly produced. However, composition alone does not explain the durability of Roman concrete.
At that time, the ingredients of concrete were also mixed using different techniques than now. One proof of that is the small, light-colored pieces of lime in the concrete. They are clearly visible in the ancient concrete.
Concrete researchers have previously believed that the pieces in the concrete were a sign of bad raw material. And because the concrete was mixed poorly, says Ny Teknik.
“I was bothered by the idea that the pieces of lime were due to poor quality control of the concrete,” says the researcher I admire Masic In the bulletin of MIT University. Masic is an associate professor of civil and environmental engineering at MIT.
Masic and his team decided to carefully study 2,000-year-old samples of Roman concrete. A scanning electron microscope and an X-ray spectroscope and other precise equipment were used.
Using spectroscopes, the researchers discovered that the light-colored pieces had formed at high temperatures.
It showed that the Romans mixed into concrete not only ordinary “burnt lime”, but also lime, which was mixed directly with pozzolan and water at a very hot temperature.
WHAT: n the group calls this type of lime “hot mix”. Its advantages are twofold, says Masic.
“When the resulting concrete is heated to very high temperatures, it allows chemistry that is not possible if you just use colder lime.”
At high temperature, new compounds are formed.
In addition, the increased temperature significantly reduces the time required for the concrete to harden, as all chemical reactions are accelerated. This is how construction speeds up.
“Hot” concrete has another important advantage. With the help of well-heated limestone, concrete can repair itself when cold.
Researchers say that it is these visible pieces of lime that make concrete self-healing.
When small cracks form in the concrete, they also spread through the fragile white stone blocks.
When water gets cracked, it reacts with lime and forms a solution containing calcium.
Then the solution dries and hardens. The substance sort of glues the crack shut and prevents the crack from spreading larger.
The cracks in the concrete mass are filled with a mixture of water-dissolved lime and various minerals. The patched crack recrystallizes, like in natural limestone rocks.
This may also explain why Roman concrete has lasted as long as 2,000 years in Italian breakwaters. After all, Mediterranean waves have been hitting concrete for centuries.
Group made concrete at MIT for comparison without that very hot burnt lime. They made cracks in it.
The quality of the hot fired cracked concrete improved in two weeks, but the concrete made for comparison had cracks, says the website Science Alert.
In Finland, and today in general, concrete is used a lot portland cement. The concrete made with it is supported by reinforcement.
This concrete is much poorer in minerals. When water finally penetrates the cracks in the concrete, it corrodes the reinforcement and increases the cracking.
Enjoy MIT:n the researchers now plan to commercialize the new kind of mixture. It would be an alternative to current concrete, and also friendlier to the environment.
Durable concrete could, for example, improve the durability of concrete structures made with 3D printing, says Masic.
He also hopes that sustainable concrete could also reduce the burden of cement production on the environment. Concrete manufacturing accounts for about eight percent of the world’s greenhouse gas emissions.
The study was published by science journal Science Advances.
Correction January 18 at 12:30 PM: Corrected the point mentioning the scanning electron and X-ray spectroscope. The right one is the scanning electron microscope.